how to change a front and rear sprockets on a 450r?

Answer 1

Always give year, make, engine size and model # when asking questions. A 450r what?
1982 Honda cr450r -
--Remove 2 bolts on F sprocket "fixing plate".
--Turn the plate to line up the splines on the counter shaft and remove it.
--With a pair of pliars, slide the clip off the chain's master link (take note as to the direction the opening of the clip points to). Remove master link. Remove chain.
--Remove R brake adjuster and unbolt brake panel from arm.
--Remove the axle and R wheel.
--Remove the R sprocket
******Very important. Follow these instructions to the letter!!!
--Clean sprocket nuts & bolts. No dirt in the heads of the allen bolts.
--Put an allen wrench or socket in the allen bolt (to hold it).
--Using a 6 point box wrench, put it on that bolt's nut.
--Hit the wrench to loosen the nut with a soft hammer (plastic or rubber malet so you don't scratch/ruin the wrench LOL).
--Then remove the nuts & bolts and sprocket.
*******If you don't remove the nuts & bolts in this manner, you'll round out the bolt's allen heads. Been there - done that.
--Put every thing back together and adjust the chain tension (get the tension specs from a shop manual).
--Anyone attempting to work on their own bike should own a shop manual. You've invested so much time and money on your precious machine, a couple of $ is well worth it. Why take the chance of getting wrong info from message boards.
http://www.motocom.com/motorcycles/
Honda crf450r is similar, except the F sprocket is held on with one bolt and it has disc brakes.
--Unbolt the caliper from the bracket and remove the caliper.

Answer 2

if you have to ask how to do something that simple don't mess with it